The use of shipping containers to house explosives or contraband presents certain detection problems, especially when the containers house non-nuclear explosives and contraband such as drugs and the like. Priorly detection of explosives or contraband in a shipping container required opening the container for inspection. This however is a cumbersome process and there is a requirement for remote substance detection that can penetrate a metal container. While various techniques may be available, nuclear quadrupole resonance has been found to work when using terminated balanced transmission lines such as described in co-pending patent application entitled Transmission Line Array For Explosive Detection Using Nuclear Quadrupole. Resonance, BAEP-1288, filed on even date herewith and incorporated herein by reference. However, the ability to use the technique to penetrate metal shipping containers was thought impossible.
By way of background, in the early 1900s, not long after Einstein published his equations on thermal equilibrium, individuals realized that there were likely to be resonances at very low frequencies for atoms and molecules and that these resonances would occur because if one emits a photon of exactly the correct frequency, the material will absorb this photon, store it for some amount of time and then get rid of the absorbed energy. It is has been found that in nature the molecules which absorb such energy always fall to a lower energy state.
One of the ways for the material to emit energy is through spontaneous emission where a photon of exactly the same energy that is impinging on the material is thrown off in a random direction at random times.
The second way of getting rid of the energy absorbed by the material is through process of stimulated emission in which a photon arrives at exactly the appropriate energy, gets near the molecule, stimulates the molecule and when the molecule drops to the lower energy state it emits a photon that is exactly in phase with the original photon.
The energy that is thrown off either in spontaneous emission or stimulated emission results in an exceedingly narrow spectral line. In fact the line is generally considered to be a single line that exists at a given wavelength or frequency. It is noted that the material only has one choice assuming that the material is pumped at its lowest energy state, raising the energy within the molecule such that the only way that it can release its energy is to emit a photon of that exact energy.
Nuclear quadrupole resonance has been utilized in the past to detect the presence of specific molecules, including explosives. Explosives generally involve the use of nitrogen or nitrogen bonded with other elements. When nuclear quadrupole resonance was utilized in the past, it was used to detect the presence of molecules due to the molecular elements that are bonded together such that the molecules absorb energy at for instance as many as eight different energy levels or spectral lines. It turns out that at least three of the energy levels tend to be prominent, although in some materials there are upwards of all eight energy levels for one bond. If one has many bonds there may be many dozens of spectral lines. In order to detect the presence of a molecule one usually is looking to pump energy right at the top of one of the spectral lines and look for energy coming back at the same frequency.
As discussed in co-pending patent application by Paul A. Zank and John T. Apostolos, entitled Method and Apparatus For Sensing The Presence Of Explosives, Contraband and Other Molecules Using Nuclear Quadrupole Resonance, BAEP-1277, filed on even date herewith and incorporated herein by reference, as part of the subject invention, it has been found that the spectral lines of interest especially for explosives are in the 100 KHz to 10 MHz range. A particularly interesting explosive is called RDX which has a spectral line in the 3 to 4 MHz range, as does sodium nitrate.
However if one is seeking to detect stimulated or emission or spontaneous emission at 3 MHz, the wavelength of the returns is incredibly long, in some cases corresponding to the size of a building. Moreover, the photons that are emitted in either spontaneous or stimulated emission represent very little energy. For instance, a red photon carries an energy of about 3.5 electron volts, with detectable radiation being one or two millionths of 3.5 electron volts. The result is that photons emitted from the molecules are virtually undetectable. One of the reasons is that in order to detect single photons one is faced with thermal background that overwhelms the detection process. In order to achieve any type of result, one pumps large numbers of photons into the target material such that for every milliwatt second an extraordinary number of photons are involved.
If the photons are at the appropriate frequency they are absorbed and only when the frequency exactly corresponds to a resonance line does the molecule start absorbing the photons. Thus it is quite important that the frequency source utilized in the nuclear quadrupole resonance measurements be extremely precise and stable.
If one performs a frequency sweep, the emission that comes back is on the order of 1% of the energy that impinges on the molecule.
It is noted that prior nuclear quadrupole resonance techniques can be likened to looking into a headlight to find a 1% response.
As a result, a pulsed coil prior art nuclear quadrupole resonance detection of molecules requires upwards of 100 kilowatts of energy coupled to a very high Q tuned coil having for instance a Q of 80 or better. If there is any offset in terms of the frequency of the incident radiation or if the coil tuning was not precise, then any emissions from the molecule will be lost in the clutter.
First and foremost in the prior art pulsed coil nuclear quadrupole resonance techniques, it was only with difficulty that one could in fact detect any response. One of the reasons is because the coil exhibits a large dwell time after which one looked for a response.
If one did not wait, the incoming radiation would swamp the detectable results. In order to eliminate this problem, those in the past used a pulsed source and then waited for a response after the trailing edge of the pulse. Prior systems thus pumped pulsed energy into a coil with the target material at the center of the coil. Thereafter the material would absorb energy and then the prior systems would listen for the spontaneous decay.
The problem with spontaneous decay that at thermal equilibrium a spontaneous photon happens only once for every two million stimulated photons. Thus, in terms of detecting spontaneous decay, one is at an extremely difficult power disadvantage. Secondly, the spontaneous decay might happen over several tens of milliseconds which means that the instantaneous power levels at any point in time are very low. For spontaneous decay using a pulsed coil nuclear quadrupole resonance, the problem is that one is working with very few photons and, further they are stretched out over time. This means that one has to use huge amounts of power to overcome these problems, often in the nature of kilowatts of energy. Moreover, because one is looking at very low signal strength the coil is made with a very high Q. This means that the coil couples well with the environment, that in turn means that the coil picks up a great deal of background noise.
Pulsed coil nuclear quadrupole resonance detection systems have been marginally cost effective and their power density has exceeded human safe limits.
More specifically, taking RDX as an example, the bandwidth of the RDX resonance is about 400 hertz. This means that the associated decay time or relaxation time is on the order of 2.5 milliseconds. If one were to sweep the frequency through the resonance as one approaches the resonant frequency, what happens is that one excites the nucleus of the nitrogen atom. When the nucleuses are excited they go into an upper state and then as one sweeps by the frequency there is a population inversion in these nuclei at which time they start to decay.
If one utilizes a long CW pulse what would happen is that one would see a periodicity of absorption and emission. When the CW pulse is turned on, the molecule goes into the excited state but then relaxes through stimulated emission. What would happen utilizing a CW signal is that one would see a series of absorptions and emissions that would occur every 2.5 milliseconds.
For RDX, assuming a pulsed coil system, one must use a pulse width of about half a millisecond because the pulse has to decay down fast enough so that the spontaneous emission can be observed.
Thus in the past a relatively short pulse of CW energy was used to enable listening for the response. However, in order to be able to detect the response at all, a very high Q coil was required. High Q coils have an excessive relaxation time. As a result, in order to provide for the ability to listen when driving a very high Q coil at half a millisecond one has to have other circuitry to quench the coil as fast as possible so as to be able to listen to the return, typically in terms of a little hiss that comes off after irradiation with the pulse.
Thus, in the prior systems one had to have exceedingly large kilowatt sources of 3 MHz energy in order to obtain enough of a response, and then had to pulse the source so as to be able to stop it and quench it in time to be able to detect the minuscule response that would occur.
Having the high Q coil further was complicated by the fact that one could not frequency sweep a sample because the high Q coil resonates at only one frequency.
This for instance precludes the ability to distinguish between the detection of multiple spectral lines to be able to distinguish the spectral response of the target molecules from the spectral responses from uninteresting molecules.
Also, when using a high Q coil one has to use an exceedingly large amount of shielding to make the system safe for use around people, as well as having to actively quench the coil.
Moreover, when pumping 1 kilowatt into a coil, the presence of the system is very easy to detect. Thus, terrorists could avoid screening knowing that such a detection system was in operation.
Note that the pulsed coil system detects spontaneous not stimulated emissions. Spontaneous emissions are not coherent and one obtains the square root of the power coming back.
Thus, in the past it has been virtually impossible to provide a workable system that would reliably and safely detect dangerous amounts of explosive material hidden on a human.